OX40 and 4-1BB downregulate Kaposi’s sarcoma-associated herpesvirus replication in lymphatic endothelial cells, but 4-1BB and not OX40 inhibits viral replication in B-cells

Middle East respiratory syndrome coronavirus-encoded ORF8b strongly antagonizes IFN-β promoter activation: its implication for vaccine design

Middle East respiratory syndrome coronavirus (MERS-CoV) is a causative agent of severe-to-fatal pneumonia especially in patients with pre-existing conditions, such as smoking and chronic obstructive pulmonary disease (COPD). MERS-CoV transmission continues to be reported in the Saudi Arabian Peninsula since its discovery in 2012. However, it has rarely been epidemic outside the area except one large outbreak in South Korea in May 2015. The genome of the epidemic MERS-CoV isolated from a Korean patient revealed its homology to previously reported strains. MERS-CoV encodes 5 accessory proteins and generally, they do not participate in the genome transcription and replication but rather are involved in viral evasion of the host innate immune responses. Here we report that ORF8b, an accessory protein of MERS-CoV, strongly inhibits both MDA5- and RIG-I-mediated activation of interferon beta promoter activity while downstream signaling molecules were left largely unaffected. Of note, MDA5 protein levels were significantly down-regulated by ORF8b and co-expression of ORF4a and ORF4b. These novel findings will facilitate elucidation of mechanisms of virus-encoded evasion strategies, thus helping design rationale antiviral countermeasures against deadly MERS-CoV infection.

Cell Type-Specific Interferon-γ-mediated Antagonism of KSHV Lytic Replication

Kaposi’s sarcoma-associated herpesvirus (KSHV) is causally associated with several malignant tumors: Kaposi’s sarcoma (KS), multicentric Castleman’s disease (MCD), and primary effusion lymphoma (PEL). KS remains the most common AIDS-related malignancy since the AIDS epidemic and thus has been extensively studied. KS is characterized as an angioproliferative disease with massive immune cell infiltration at the early stage. High levels of proinflammatory cytokines and growth factors are found in KS lesions, and their involvement in the survival and growth of tumor cells has been well characterized. However, little is known about the role of the inflammatory microenvironment in the regulation of KSHV gene expression and/or viral replication. In the present study, we demonstrated that IFN-γ and TNF-α profoundly inhibited KSHV progeny production in primary human lymphatic endothelial cells (LECs) as well as induced KSHV-producer cells (iSLK.219) with doxycycline. Of note, IFN-γ inhibited overall KSHV gene expression, while the effects of TNF-α were confined to a selected set of genes, which were also downregulated by IFN-γ. The addition of IFN-γ up to 36 hr after induction of viral lytic replication was effective in terms of the inhibition of infectious virion production, suggesting that its inhibitory effect is exerted at the early stages of KSHV life cycle. We believe these data have potentially important implications for rationalizing a therapeutic agent to treat KSHV-induced tumors in which lytic replication plays a critical role in their pathogenesis: KS and MCD.

Primary lymphocyte infection models for KSHV and its putative tumorigenesis mechanisms in B cell lymphomas

Kaposi's sarcoma-associated herpesvirus (KSHV) is the latest addition to the human herpesvirus family. Unlike alpha- and beta-herpesvirus subfamily members, gamma-herpesviruses, including Epstein-Barr virus (EBV) and KSHV, cause various tumors in humans. KSHV primarily infects endothelial and B cells in vivo, and is associated with at least three malignancies: Kaposi's sarcoma and two B cell lymphomas, respectively. Although KSHV readily infects endothelial cells in vitro and thus its pathogenic mechanisms have been extensively studied, B cells had been refractory to KSHV infection. As such, functions of KSHV genes have mostly been elucidated in endothelial cells in the context of viral infection but not in B cells. Whether KSHV oncogenes, defined in endothelial cells, play the same roles in the tumorigenesis of B cells remains an open question. Only recently, through a few ground-breaking studies, B cell infection models have been established. In this review, those models will be compared and contrasted and putative mechanisms of KSHV-induced B cell transformation will be discussed.